This invention relates to an improved process for the purification of polycarbonate containing solutions obtained in a two phase interfacial synthesis process. The process provides for the use of at least one plate decanter to separate and purify the polycarbonate containing solution.
There are many well known processes for the preparation of polycarbonates from monomers. A preferred method is a two-phase interfacial process in which phosgene is contacted with aromatic bisphenols and condensed to produce high molecular weight polycarbonates.
The two-phase interfacial process results in a reaction mixture which is an emulsion of organic and aqueous alkaline phases. In order to recover the polycarbonate product from the organic phase in which it is found, the reaction mixture must first be separated into its organic and aqueous alkaline phases, and the organic phase must be further processed to remove undesirable impurities including electrolytes, catalysts, residues of chain terminators, and unreacted starting materials.
Typically, such reaction mixtures contain polycarbonate polymers with molecular weights up to 250,000 g/mol on a polycarbonate basis and brine present in an amount around one quarter by weight of the reaction mixture. Further, the reaction mixtures may exhibit a wide range of viscosities. In conventional processes, the initial separation of the aqueous alkaline phase from the organic phase is not complete, but leaves the organic phase containing some ten percent or more of an aqueous component. Further, the organic phase contains residual impurities such as catalysts, residues of chain terminators, and unreacted bisphenols. Thus, additional steps are required to purify the organic phase.
For example, U.S. Pat. No. 4,338,429 describes removing alkali metal hydroxide, catalyst, and salts from the organic phase by repeated settling or centrifugation, with the addition of either aqueous acid or water in each step. The process is enhanced by the addition of shear energy and/or cationic emulsifying agents and dispersing agents to the aqueous acid step, or water-soluble organic anionic compounds to the water step.
Another method for purifying the organic phase is coalescence, as described in U.S. Pat. No. 4,316,009 and German Patent DE 19510061. The continuous process described therein involves washing and reemulsification, followed by reseparating the two phases by conveying the emulsion through a layer of fibers that causes coalescence of the separate phases.
Another method for purifying the organic phase is to employ a series of centrifuges to remove the residual aqueous component by application of very high gravitational forces, as described in U.S. Pat. No. 5,260,418. In this process, the separated organic phase is further contacted with water and various aqueous solutions to bring the impurities into the aqueous component, which is removed and discarded.
Phase separation and purification processes employing centrifuges are time consuming and expensive to install and operate. Moreover, alternative purification methods such as extraction by countercurrent water washing and gravity separation of the two phases are much slower and less efficient. Accordingly, there is a need to provide a method for the separation and purification of reaction mixtures resulting from the two-phase interfacial process for making polycarbonate which is efficient and economical.
Therefore, it is an object of the present invention to provide an economical and efficient process for the separation and purification of a reaction mixture obtained in a two-phase interfacial process for making polycarbonates.
These and other objects of the invention are obtained by introducing the reaction mixture into a separation device comprising at least one plate decanter thereby separating the reaction mixture into an organic phase and an aqueous phase. The separation may be carried out using a combination of plate decanters, coalescers decanters, and centrifuges, which are operated in series or in parallel. Plate decanters in combination with such other devices provide efficient and effective separation of the reaction mixture into an organic phase containing the polycarbonate, and an aqueous waste phase.
In one embodiment, two plate decanters are operated in series. The plate decanters in series may precede or follow other separation devices, or may receive reaction mixture effluent directly from a reactor.
In an alternate embodiment, the separation devices are combined in a plate decanter/coalescer decanter array comprising a plurality of units comprising at least one plate decanter. The array comprises m rows where each row consists of n units in series, in which the organic phase output of each unit is connected with the outputs of units having the same value of n in other rows. The organic phase outputs of the last unit of each row of the array are combined thereby separating a reaction mixture into an organic phase and an aqueous phase. The organic phase may be conveyed into a series of centrifuges, and then into another array in a repetitive process.
By the process of this invention it is possible to separate and purify the reaction mixture in a highly efficient and economic manner.